Project Summary/Abstract Human clinically non-functioning pituitary tumors of a gonadotroph cell origin, represent up to 30% of all diagnosed pituitary neoplasms. These tumors cause considerable morbidity because of mass effect, resulting in neurologic deficits and the development of hypopituitarism. Unfortunately, there are no established adjunctive medical therapies to control tumor growth, and the mechanisms for the pathogenesis of such tumors remain largely unknown. Therefore, there is a critical need to identify molecular events specifically linked to the development of these tumors. A major finding by our group regarding the pathogenesis of human pituitary tumors is that such tumors are monoclonal in origin, so that a somatic mutation is a requisite event in tumor formation. The next critical questions are what mechanisms underlie selective clonal proliferation in pituitary tumors, as well as the specific development of the clinically nonfunctioning tumor phenotype. We have identified MEG3, a novel pituitary-derived gene which inhibits tumor cell proliferation. Its mouse homolog is Gtl2/Meg3. MEG3 gene expression is lost in human clinically nonfunctioning pituitary tumors as well as other human cancer cells. Most importantly, unlike other tumor suppressors, MEG3 gene products function as a non-coding RNA, and they functionally interact with tumor suppressors p53 and Rb to inhibit tumor growth. Therefore, we hypothesize that MEG3 plays a critical role in preventing human tumor development via unique and entirely novel mechanisms. The goal of our study is to discover these mechanisms. In this grant application, we propose to 1) investigate molecular mechanisms by which MEG3 non-coding RNA suppresses growth of pituitary tumors, 2) explore the structure-function relationship of MEG3 non-coding RNA, and 3) study the in vivo function of the Gtl2/Meg3 gene in mouse embryonic development and cellular transformation using gene-targeting techniques. The discovery of a non-coding RNA gene linked to other major tumor suppressors is very exciting. The study of MEG3 will provide important information regarding the pathogenesis of human pituitary adenomas; reveal novel mechanisms to broaden our knowledge of the involvement of large non-coding RNAs in human cancer biology; and point out new directions for the potential development of novel and effective therapeutic strategies for human pituitary tumors.